Identification of biosecurity measures and spatial variables as potential risk factors for Aleutian disease in Danish mink farms

Abstract

Eradication of Aleutian disease was initiated in Denmark in 1976. The prevalence of positive farms has since then been reduced from 100% to only being continuously present in the region of Vendsyssel, Northern Denmark since 2004. In this study, we attempted to identify risk factors for the infection in this region based on logistic regression of spatial (environmental, neighbourhood) variables and biosecurity measures. Information on potential biosecurity (management) risk factors in the region was obtained from interviews in 342 registered farms in the region using a structured questionnaire. A total of 279 questionnaires were completed (response rate 82%). Additional spatial variables were included in the analysis. The study shows that farm size (the number of animals in the farm) and proportion of infected neighbouring farms were significant risk factors for infection with Aleutian Mink Disease Virus. These factors account for 35% of the variation of the infection status of mink farms located in Vendsyssel during 2009. These results indicate that only a coordinated effort from the farmers in the area will succeed in eradicating the disease from Denmark, because individual farms that have eradicated the disease will be at risk of re-infection from test-positive neighbours.

Introduction

Mink plasmacytosis or Aleutian mink disease (AD) is an infectious disease caused by the Aleutian Mink Disease Virus (AMDV). The disease causes reduced litter sizes, embryonic death, stillbirth and abortions, fur damage and eventual death due to a heavy antibody response to the presence of the virus (Broll and Alexandersen, 1996). Because of its severe economic consequences, Denmark has tried to eradicate it at a national level from mink farms since 1976 (Chriél, 2000). At that time there were 2423 mink farms in Denmark, which has by 2009 been reduced to 1475; 342 of these are located in Vendsyssel, Northern Denmark. The average number of mink produced during the 2009 season (April to December) per farm was 1850 in the whole country and 1574 in Vendsyssel.

The surveillance part of the eradication program consists of mandatory testing. Farms outside the control zone (an area where the eradication effort is currently focused on, with animal movement and trade restrictions in place) must test a portion of the breeding animals from May to June and again between November and January before the mating period, while infected farms must test all breeding animals 3–6 times per year. The blood samples are tested by the counter immunoelectrophoresis test. Most farmers will stamp out the disease by culling all mink if they get a positive test, although it is not mandatory. Others choose to cull all the animals except the breeding stock which will be used to breed the next generation of animals (Mariann Chriél, personal communication). Farms that present more than three positive samples during a test are considered infected and farms with up to three positive samples are considered suspect. Transportation of animals from suspected and infected farms is restricted.

The national program has been successful to the extent of reducing the percentage of farms with AD from 100% in 1976 to 15% in 1996 (Aasted et al., 1998). Measures were initially voluntarily adopted by mink farmers. However, eradication progress declined in the late 1990s with ca. 10% farms still infected with the disease. Therefore, the first public act was put in force in 1999 and has been updated several times since then. In 2001, the percentage of infected farms was 5% (Christensen et al., 2011). The demands for improved biosecurity and disease control zones are in focus with the ultimate goal to eradicate the disease from Denmark. Since 2004, infected farms have been restricted to the region of Vendsyssel in Northern Denmark. A study by Espregueira Themudo et al. (2011) shows that the spatio-temporal distribution of AD in Denmark confirmed that Vendsyssel has persistent spatial clusters of AD, and that the farm level prevalence of the disease in these clusters is significantly higher than in the remaining of Vendsyssel. Special measures have been put in place to increase the efficiency of the eradication program, and since January 2010 a disease control zone was established around the affected area (Vendsyssel). Farmers in Vendsyssel are at present not allowed to sell animals to farms outside the disease control zone.

Studies on the strain diversity and stability of the causal agent, Aleutian Mink Disease Virus, by Christensen et al. (2011) showed that Danish isolates represent a very homogenous cluster compared with Swedish, Finnish and Dutch isolates. It seems to represent a minor fraction of the genetic diversity previously found in Denmark. This indicates that eradication progress exceeds the rate of mutation driven diversity. The finding of similar strains in the consecutive year after infection indicated disinfection procedures on infected farms not to be adequate and/or lack of biosecurity resulting in re-infection from other farms.

As an alternative to culling, vaccination has been considered as a control strategy. However, Aasted et al. (1998) found that vaccines based on capsid proteins enhanced disease (increasing the mortality rate) while vaccination with major non-structural AMDV protein only provided partial protection from AD. More recently Castelruiz et al. (2005) developed a DNA vaccine based on the gene encoding for AMDV non-structural protein 1 (NS1). They concluded that the vaccine showed partial protection with milder disease characteristics in the vaccinated mink.

It has been suggested that farmed mink can be infected from infected wild or feral mink or vice versa. Fournier-Chambrillon et al. (2004) proved that AD is present in free ranging mink by detecting AMDV antibodies in free-ranging European mink and other small carnivores in Southwestern France while Farid et al. (2010) and Nituch et al. (2011) had similar results in free ranging American mink in Canada. Danish studies from 1998–2001 showed 11% of 45 trapped free ranging mink to be test positive in the region of Vendsyssel in Northern Denmark (Willadsen and Dietz, 2009).

Control efforts in other mink producing countries are very sparsely described in the literature. Cho and Greenfield (1978) reported that Aleutian disease was eliminated from three Canadian mink farms by culling all test positive mink (a la test and slaughter) during the pelting season and before the breeding season for four consecutive years. Gunnarsson (2001) reported that Iceland successfully eradicated AD from farmed mink by pelting all mink on test positive farms (i.e. the stamping out method) during 1982–1984, reporting only a single case in 1996.

The objective of the present study was to identify risk factors for infection with AMDV in mink farms located in Vendsyssel. Focus was on spatial (neighbourhood and environmental) variables as well as information on biosecurity measures extracted from a questionnaire directed at mink farmers of the region.